Selenium has three allotropes (see allotropy),
the most stable being the gray, metallic form. Its chemistry is analogous
to that of sulfur. It is used to make photoelectric
cells, solar cells, and rectifier,
in xerography, and as a semiconductor;
also to make ruby glass and to vulcanize rubber.
Its most common isotope is 80Se (49.82%).

atomic number

34

relative atomic mass

79.0

relative density

4.79

melting point

217°C (423°F) (gray)

boiling point

685°C (1,265°F) (gray)

Selenium and life

Selenium plays an important role in the metabolic functions of the body
and is essential for maintaining optimum health in humans and other animals.
It is an important antioxidant and is
significant in cell formation, healthy reproductive processes, and adequate
immune response. However, even small amounts of selenium above normal levels
can lead to chronic or acute toxicity in humans and other animals and can
have a destructive effect on the environment. The riches dietary sources
of selenium are meat, fish, whole grains, and dairy products.